In addition to transcriptional regulation of gene output, mammalian genomes produce extensive transcriptome and proteome diversity by alternative splicing and selection of alternative 3' mRNA ends during pre-mRNA processing. As for transcription, transcriptome processing is extensively regulated in response to dynamic physiological needs. The regulation of transcriptome processing involves interconnected networks controlled by RNA binding proteins that bind to preferred sequence motifs within the pre-mRNA near the sites of regulated processing. The long term goal of this project is to determine the extent, regulatory mechanisms, and functional consequences of transcriptome processing in adult skeletal muscle. The disruption of transcriptome processing networks contributes to disease in skeletal muscle yet little is known regarding the extent or functions of normal regulation. In the first part of this proposal, we will identify the regulatory networks controlled by the Fox family of RNA binding proteins in skeletal muscle and determine their functions during myoblast differentiation. We will use tissue specific and inducible knock outs of the two Fox genes expressed in skeletal muscle to determine the functions of the regulatory networks in myofibers and satellite cells in adult skeletal muscle. In the second part of the proposal, we will identify additional regulatory networks operative during myoblast differentiation using a bichromatic splicing reporter in high throughput RNAi screens. Knowledge gained will be directed toward understanding the roles of these networks in adult skeletal muscle. The results will provide a new understanding of the role of nuclear post-transcriptional regulation in the diverse homeostatic functions of adult skeletal muscle and its capacity for repair. This understanding is important for development and application of novel therapeutic strategies to conditions that negatively affect skeletal muscle function.